Forging mandrel for hot-forging tubular workpieces of metal

ABSTRACT

The invention relates to a forging mandrel for hot-forging tubular workpieces of metal, said mandrel comprising a mandrel body and a mandrel bar, in which forging mandrel the mandrel body is composed of a heat-resistant material. The forging mandrel according to the invention is embodied here in such a way that the mandrel body is provided, at least in the work zone, with a layer which reduces the dissipation of heat into the mandrel body during the forging process and adheres firmly to the mandrel body, wherein the layer has a lower thermal conductivity than the material of the mandrel body and has a thickness of at least 50 μm.

The invention relates to a forging mandrel for hot-forging tubular workpieces of metal according to the preamble of patent claim 1.

Hot-forming of tubular workpieces, such as, e.g., seamless steel pipes, is known from DE 10 2005 052 178 A1.

Starting from a block heated to forming temperature, this method involves a first forming step in which a thick-walled hollow block is produced through punching and then forged into a seamless pipe by a radial forging process which uses a forging mandrel as internal tool which is pushed into the hollow block and at least two forging jaws of a forging machine to act on the outer surface area of the hollow block. The hollow block is hereby turned and shifted axially in clocked manner in the phase of the idle stroke of the forging jaws.

The forging mandrel is exposed during the forging process to significant thermal and mechanical stress, causing wear of the tool surface and decreasing the service life of the forging mandrel. Forging temperatures of above 1300° C. may occur for example.

In order to minimize wear caused as a result of thermal exposure, it is known from DE 2825940 A1 to cool the forging mandrel from the inside in order to accelerate heat dissipation from the surface of the mandrel into the mandrel.

In order to increase wear resistance of the mandrel, it is further proposed to surround the mandrel body with an outer jacket of highly heat-resistant material, e.g. a nickel or cobalt base alloy.

This forging mandrel suffers several drawbacks. On the one hand, this forging mandrel is very complex and expensive to manufacture. On the other hand, heat introduction into the mandrel material is not diminished so that the shape stability of the forging mandrel is reduced, when exposed to high temperature stress and thus causes premature failure.

It is an object of the invention to provide a forging mandrel for hot-forging tubular workpieces of metal, which forging mandrel can be manufactured inexpensively and exhibits high wear resistance while yet is of stable shape even when exposed to high forging temperatures.

Starting form the preamble, this object is solved in combination with the characterizing features of claim 1. Advantageous refinements are the subject matter of sub-claims.

According to the teaching of the invention, the mandrel material is made of heat-resistant alloy, with the mandrel body provided at least in the work zone with a layer which lessens heat dissipation into the mandrel body during forging and adheres firmly upon the mandrel body and which has a thickness of at least 50 μm.

The essential novelty of this invention resides in that the layer which surrounds the mandrel body has a lower thermal conductivity than the mandrel body so that heat introduction into the mandrel body is significantly decreased to thereby realize a significantly improved shape stability, even when exposed to high forging temperatures.

The layer has hereby in accordance with the invention a heat conductivity which is significantly lower than the one of the material of the mandrel body, advantageously by at least 50%.

The layer on the mandrel body can be advantageously produced according to the invention in a very cost-effective way through oxidation or also through application of a respective layer, e.g. through thermal methods.

Oxidizing the mandrel is advantageously applied when carbon steels are involved, whereas high-alloy mandrel materials involve application of the layer advantageously through thermal spraying processes, such as, e.g., flame-spraying or plasma spraying.

The thermally applied layers may hereby contain zirconium oxide, aluminum oxide, or iron oxide, for example.

It is hereby essential that the thermal conductivity of the coating material is always significantly below the one of the mandrel material.

In order for the layer to be able to exert a recognizable effect with respect to a significantly reduced heat introduction into the mandrel body, it is advantageous when the layer has a minimum thickness of about 50 μm.

The forging mandrel according to the invention has hereby not only the advantages with respect to improved shape stability but also with respect to a decrease of the abrasive wear. Tests have shown the presence of a certain lubricating effect of the layer material during forging to reduce the abrasive wear.

For selecting a suitable material for the mandrel body, it is essential to suit the mandrel body to the material being forged in such a way that fusion of forging mandrel and forging workpiece is prevented.

The forging mandrel according to the invention has hereby the beneficial effect that the layer acts upon the mandrel body as a separation layer which opposes a fusion under forging pressure and temperature.

As a certain heat introduction into the mandrel body cannot be completely eliminated, it may be further prudent to provide the mandrel body with an additional internal cooling in order to assist the shape stability of the mandrel.

As an alternative, it may also be provided to cool the outside of the mandrel body in phases of standstill. This can be implemented in a much more cost-efficient manner than internal cooling.

In order to limit the heat introduction into the mandrel body to a minimum during the forging process, it may be further prudent to move the mandrel in the workpiece during forging because in this procedure constantly other work zones of the forging mandrel surface come into engagement so that local heat introduction is reduced.

The forging mandrel according to the invention will be described in greater detail with reference to an illustration.

The forging mandrel shown in the sole FIGURE has a mandrel body 1 provided with bevels 3, 4 and secured to a restraining bar 2. The restraining bar 2 is required to be able to move the forging mandrel in axial and rotatable relationship in a tubular workpiece, not shown here.

The forging mandrel 1 is made of heat-resistant steel which is provided in accordance with the invention with a layer 5 which reduces in the work zone the heat introduction into the mandrel body 1.

The layer 5 has hereby a thermal conductivity which is significantly lower than the one of the material of the mandrel body 1 in order to reduce heat introduction and thus to ensure the shape stability of the mandrel body 1, even when exposed to high forging temperatures.

The mandrel body 1 is configured as solid body in the present case; it is, however, also possible to provide the mandrel body with internal cooling to dissipate as rapidly as possible unavoidable heat introduction into the mandrel body 1 during forging.

LIST OF REFERENCE SYMBOLS

No. Designation 1 Mandrel body 2 Restraining bar 3, 4 Bevel 5 layer 

1.-6. (canceled)
 7. A forging mandrel for hot-forging a tubular workpiece of metal, comprising: a mandrel body made of heat-resistant material; and a layer adhering firmly upon the mandrel body at least in a work zone thereof, said layer being configured to reduce heat dissipation into the mandrel body during forging and having a thermal conductivity which is lower than a thermal conductivity of the material of the mandrel body, said layer having a thickness of at least 50 μm.
 8. The forging mandrel of claim 7, wherein the layer is an oxide layer.
 9. The forging mandrel of claim 7, wherein the layer is applied onto the mandrel body through a flame-spraying process.
 10. The forging mandrel of claim 7, wherein the layer is applied onto the mandrel body through plasma spraying.
 11. The forging mandrel of claim 7, wherein the thermal conductivity of the layer is lower by at least 50% than the thermal conductivity of the material of the mandrel body.
 12. The forging mandrel of claim 7, wherein the mandrel body is constructed in the form of a hollow body which is provided with an opening on one side and includes an internal cooling device which extends at least into the work zone.
 13. The forging mandrel of claim 7, wherein the mandrel body has a mandrel bar to allow displacement of the mandrel body within the workpiece in axial and rotatable manner. 